1. Field of the Invention
The present invention relates to a decorative coating that is formed on a surface of a resin substrate that is placed on a path of electromagnetic waves of a radar device.
2. Description of Related Art
The antennae of a communication device or radar that sends and receives radio waves have almost no limitations on its design and the structure around it because a priority is placed on its function. For example, as the antenna for a vehicle radio, a barely-formed rod antenna is used. In some cases, an antenna is wanted to be placed in an out-of-sight location depending on where it is attached. For example, a radar which measures the distance to an obstacle ahead of the vehicle or the distance to the preceding vehicle is preferably placed at the center of the front of the vehicle for its best performance. In such a case, the antenna is located in the vicinity of the front grill of the vehicle, for example, and is preferably not visible from the exterior for design reasons.
An auto-cruise system, which is a technology that measures the information such as the distance or relative speed to the preceding vehicle with a sensor mounted on a front part of a vehicle and controls the throttle or brake based on the measured information to accelerate or decelerate the vehicle to control the distance to the preceding vehicle, is attracting attentions in recent years as one of intelligent transport system (ITS) technologies which aim at relieving traffic congestion and reducing traffic accidents. As a sensor for the auto-cruise system, a radio wave transmitter-receiver, such as millimeter-wave radar, is usually used.
A radar device which is mounted on a front part of a vehicle body is usually located behind the front grill, however, the emblem of the manufacturer of the vehicle and ornaments that are specially designed for the vehicle are usually attached to the front grill. The millimeter waves from the radar device is directed forward through the front grill and emblem and reflected by an object such as a preceding vehicle or an obstacle ahead, and the reflected waves return to the radar device through the front grill. Thus, it is preferred that a material or paint which causes little radio-wave transmission loss and can provide desired good appearance is used for the portions of the front grill, emblem, etc. which are placed on a beam path of the radar device.
For the above reasons, front grills usually have a window through which radio waves can pass at a position corresponding to the location of the radio wave transmitter-receiver so that radio waves can travel through the window. On the other hand, however, the window breaks the continuity of the appearance of the front grill, and may spoil the appearance of the vehicle because it makes the radio wave transmitter-receiver or engine room in the vehicle visible from the exterior. Thus, a cover (decorative coating) which is transmissive to radio waves is usually inserted in the window of the front grill so that the window and the front grill appear consistent as disclosed, for example, in Japanese Patent Application Publication No. 2000-159039 (JP-A-2000-159039).
This is explained with reference to FIGS. 6 and 7. As shown in FIG. 6, a radar device D which is provided on a front part of a vehicle body A is located behind a front grill F. As shown in FIG. 7, the millimeter waves from the radar device D (millimeter waves L1) are directed forward through the front grill F or an emblem E and reflected by an object, such as a preceding vehicle or an obstacle ahead, and the reflected waves (millimeter waves L2) return to the radar device D through the emblem E or the like. A decorative coating is provided on the emblem E.
There are two embodiments, as shown in FIGS. 8A and 8B, of the decorative coating which is provided on the reverse side of the front grill F and on the path the electromagnetic waves of the radar device. A decorative coating M that is shown in FIG. 8A is composed of an organic substance layer Y in which nanoparticles P of a metal material, such as gold or an alloy thereof, silver or an alloy thereof, tin or an alloy thereof, or indium or an alloy thereof, are dispersed.
A decorative coating M′ that is shown in FIG. 8B schematically illustrates the decorative coating that is disclosed in JP-A-2000-159039, and includes metal nanoparticles P′ of indium or the like that are vapor-deposited discontinuously on a surface of a front grill F, and an organic substance layer Y that is formed over the metal nanoparticles P′.
Such a decorative coating is usually evaluated in terms of its performance by a weathering test, such as xenon accelerated lightfastness and weathering test. According to the inventors, when metal nanoparticles are protected by an organic substance layer as shown in FIGS. 8A and 8B, the organic substance layer is easily deteriorated by the thermal energy that is emitted from an arc lamp during the weathering test, and new problems that are associated with the deterioration of the organic substance layer, such as oxidation of the metal nanoparticles, increased susceptibility to deterioration of the decorative coating due to the oxidation of the metal nanoparticles, and change in the color tone of the decorative coating, have been found.